Typically, most integrated circuits are comprised of a multitude of transistors and the circuit typically comprises a number of stages, for example, an input stage, a emitter-follower stage, etc. The type of transistor used, for example FET (Field Effect Transistor), BJT (Bipolar Junction Transistor), etc., will often depend on the design considerations and the intended application of the integrated circuit.
Most active circuits would comprise a so-called biasing circuit, which is used to set the integrated circuit to operate at a desired quiescent operating point depending on the design requirements. For example, it might be required to bias a FET transistor to operate with a drain-source voltage (VDS) of 4 Volts and at 60% of the saturated drain current (IDSS).
The choice of bias circuit used is determined by, amongst other things: bias level, precision, stability, etc. There are many different ways to create such a bias circuit.
FIG. 1 shows a commonly used bias circuit known as a PTAT (Proportional To Absolute Temperature) generator. The circuit in FIG. 1 has first 100 and second 200 branches connected between supply VDD and ground GND rails. The first branch 100 comprises a first bipolar transistor Q1 with its base tied to its collector, a second bipolar transistor Q4 and a resistor R. The resistor R is connected to ground at one end and to the emitter of the second transistor Q4. The collector of the second transistor is connected to the collector of the first transistor Q1. The emitter of the first transistor is connected to the voltage supply VDD.
The second branch 200 includes a third bipolar transistor Q2 with its base connected to the base of the first bipolar transistor Q1 in the first branch, and a fourth bipolar transistor Q5 with its base connected to its collector and its base also connected to the base of the second bipolar transistor Q4 in the first branch. The emitter of the fourth transistor Q5 is connected to ground and the collector of the fourth transistor is connected to the collector of the third transistor Q2. The emitter of the third transistor is connected to the voltage supply. Thus, the first and third transistors (Q1, Q2) are connected in a current mirror configuration, as are the second and fourth transistors (Q4, Q5). An output transistor Q3 is located in a third branch 300 with its base connected to the bases of the first and third transistors Q1,Q2 and its emitter connected to the supply rail VDD. The output current Iout is the collector current of the output transistor Q3 which is supplied to the load driven by the output current. The emitter of the second bipolar transistor Q4 in the second branch is connected to the lower supply rail GND through the resistor R. The first, second and third branches are connected in parallel.
In this circuit assuming the area of the bipolar transistor Q4 is n times the area of the bipolar transistor Q5 then it can be shown that the output current IOUT is given by:
      I    OUT    =                    V        T            ⁢              ln        ⁡                  (          n          )                      R  
Where VT is the thermal voltage (KT/q) and ln(n) is the natural logarithm of n. Hence IOUT is proportional to the absolute temperature T.
However, the disadvantage of the biasing circuit of FIG. 1 becomes evident when the circuit is to be put into a so-called “standby” mode of operation. In standby mode, the bias circuitry is in effect bypassed and no bias values are produced. This would typically entail the use of a MOSFET switch (not shown), which is located between the bases of the second and fourth transistors Q4 and Q5 and ground GND. For example, a MOS transistor could be connected so that: its drain terminal is connected to the base terminal of the Q4 transistor, its source terminal is connected to the emitter terminal of Q5, and its gate terminal is connected to an IN terminal (not shown), which provides a signal for turning the MOS on or off.
However, the disadvantage of using a MOS device within a circuit of this type is that it the gate source breakdown voltage of MOS devices is considerably lower than that of bipolar devices and the required operating voltage of the integrated circuit. Therefore, a MOS switch cannot be adequately used when relatively high voltages are used.
It is an object of an embodiment of the present invention to have a bias circuit, which can operate in a standby mode at relatively high voltages without being susceptible to the aforementioned disadvantages.